The present invention relates to radio frequency circuits and their methods of operations, and more particularly to circuits which control the tuning ranges of VCOs.
Voltage controlled oscillators (VCOs) play an important role in many of today's communications electronics, particularly RF frequency receivers. VCOs are typically used in phase locked loops which allow RF frequency receivers, such as radios, cellular telephones, cable modems, televisions, etc., to receive large amounts data at high speed and with little error. As known in the art and used herein, the term “VCO” applies synonymously to a voltage controlled oscillator circuit, as well as other components, such as capacitors, varactor diodes, and inductors, which can be used with a VCO to control the VCO's tuning range.
As communication bands have broadened (e.g., the television band presently extending from 50 MHz to 890 MHz) it has become necessary to use multiple VCOs to accurately tune to and receive data communicated on these bands. Multiple VCOs have been successfully implemented in phase locked loops to extend the tuning range of receivers in the aforementioned applications.
While multiple VCO systems have been used successfully in various systems, some limitations remain in the VCO control circuitry. One limitation, slow tuning speed, comes as a result of how tuning ranges are selected. Often times in conventional phase locked loop systems, each VCO tuning range is selected and monitored to determine if its selection results in the phase locked loop locking onto the desired frequency. If the selected tuning range does not result in a lock, another tuning range (which may be provided by the same VCO or a separate VCO) is selected and the loop again checked for lock. The process continues for each tuning range, each selection resulting in waiting for the loop to settle in order to determine if a locked condition exits. In a multiple VCO system in which the last tuning range provides lock, the phase locked loop's settling time, and accordingly the receiver's tuning time is slowed considerably. Slow tuning time significantly reduces the performance of a receiver, as channels cannot be changed quickly and receiver tuning can be easily lost. Slow tuning time is especially detrimental in receivers which communicate data using frequency agile modulation techniques, as slow tuning speeds severely reduce or completely prevent operation.
Another limitation in the control circuit of conventional multiple VCO systems, is the lack of variability in delay time needed when checking a selected VCO tuning range for a locked condition. In a phase locked loop, for example, the VCO control circuit activates a VCO to generate a specific tuning range, waits a predetermined period (the phase locked loop's settling time), and monitors the loop for a locked condition. If the control circuit does not detect a locked condition, it controls the tuning circuitry (a VCO, arrangement of capacitors, varactor diodes, inductors) to select another tuning range and repeats the process.
The settling time for phase locked loops are mostly dependent upon the loop's bandwidth, which may vary greatly depending upon the specific application. Narrow band phase locked loops will have relatively long settling time and correspondingly long delay times between VCO activation and lock monitoring, whereas wider band loops will exhibit relatively shorter settling times and require a shorter delay period between VCO activation and lock check. As a consequence of the difference in required delay times, the conventional approach has been to build different phase locked loops for different loop bandwidths. As phase locked loops are highly integrated, it would be more advantageous to build a single system which could be used with application having differing loop bandwidth requirements.
What is therefore needed is an improved VCO control circuit and method of operation which can more quickly select the proper VCO tuning range and which can be broadly used, for instance in applications having different loop bandwidth requirements.